A variety of automatic vehicle navigation systems has been developed and used to provide information about the actual location of the vehicle as it moves over streets. For example, one general approach to such vehicle navigation systems is known as "dead reckoning", in which the vehicle is tracked by advancing a dead reckoned position from measured distances traveled and headings of the vehicle.
Central to all of the dead reckoning navigation systems for vehicles is the need to generate data regarding the heading of the vehicle as it travels through constantly changing conditions. Any error in the heading information generated by the system translates directly into an error in the positioning of the vehicle for dead reckoning type systems.
Some prior art systems employ an absolute heading sensor, such as a magnetic compass or gyrocompass, to generate data regarding the heading of the vehicle. Gyrocompasses are expensive, and inappropriate for use in vehicles, such as automobiles or other land vehicles, that rapidly maneuver. Magnetic compasses also do not work ideally for land vehicles because for instance, the streets may be surrounded with large steel structures which cause anomalies in the magnetic field of the earth around the structures which will in turn deflect a magnetic compass reading as the vehicle drives past. Further, the roads may be banked, inclined, or crowned and thereby cause other errors, such as magnetic dip error, which are reflected in the signals generated by the sensor. If the magnetic compass is gymballed, accelerations of the vehicle affect the alignment of the compass and thereby create apparent magnetic dip error.
Another type of direction sensor measures relative heading. Examples include directional gyro compass, gas turning-rate sensor, laser ring gyro compass, vibrating rod turning-rate sensor, and differential odometer. This sort of sensor measures turning rate from which the relative heading of the vehicle is calculated at a given time given the heading at a prior time. Relative heading sensors are likewise subject to a variety of errors caused by temperature, component drifts, and component offsets. Differential odometers are subject to errors due to characteristics of the streets, the wheels of the vehicle or the vehicle itself, such as crowning of the roads, bumpy roads, uneven ride of the wheel over the streets, misalignment of the wheels, and so on. For these and other reasons, the accuracy of this sort of sensor is difficult to maintain.
For any given type of heading sensor, the sources of errors in the signal generated are for a large part external to the sensor itself, so calibration of the sensor cannot compensate for some sources of heading inaccuracy in land vehicles. Because all heading sensors are subject to errors caused by some combination of the conditions of the vehicle, of the sensor and of the streets or terrain over which the vehicle travels, prior art navigation systems have been unable to generate accurate data regarding the heading of a land vehicle.
Accordingly, there is a need for an apparatus for generating accurate data regarding the heading of a land vehicle as it travels through constantly changing conditions, such as when the land vehicle travels over varied surfaces and through varying environments.